The theme of this program has not changed in 15 years: Understanding the biology of human hematopoietic stem cells (HSC) and their progeny will result in improved hematopoietic cell-based therapy for a variety of lethal malignant diseases. In the current funding period we have established double umbilical cord blood (DUCB) transplantation as an effective treatment which may transform the practice of hematopoietic cell transplantation (HCT) because it vastly increases the pool of patients to whom transplant can be offered. We will now approach three important issues in the DUCB transplant setting-graft versus host disease (GVHD), delayed immune reconstitution with resultant late infection, and refractory or relapsed leukemia. John Wagner MD and his co-investigator Bruce Blazar MD have generated preclinical data demonstrating the suppressive effect of UCB-derived regulatory T cells (Treg) on GVHD and performed first-in-human UCB Treg safety and dose-finding trials. In Project 1, Dr Wagner proposes a series of clinical trials testing the efficacy of UCB Treg to prevent and to treat acute GVHD including add-back of UCB Tregs and effector T cells (Teffs) in calibrated doses, and development of off-the-shelf UCB Treg products. Dr Blazar has characterized UCB-derived progenitor T cells (Tprogs), and in Project 2 proposes basic studies exploring their role in restoration of thymic epithelial cell (TEC) function as well as inducible pluripotent stem cell (iPS) models to replace TEC. Findings from these studies will be translated in clinical trials conducted in Project 1 assessing the safety and efficacy of UCB Tprog therapy to reconstitute immune function following transplant and to reduce late, intracellular infections. Finally, in studies supported by this program, Jeffrey Miller MD has confirmed the marked anti-leukemia effects of allogeneic natural killer (NK) cells. In Project 3 he proposes pre-clinical and clinical studies of haplo-identical NK cell adoptive therapy used in combination with DUCB transplant to provide both immediate tumor reduction and long-term anti-leukemia effects in patients with refractory or relapse acute leukemia. These interactive projects are supported by administrative and biostatistical cores (A and B), as well as Core C, providing cGMP cell processing and immune monitoring and Core D, providing animals, environment and expertise to support human adoptive transfer experiments. This long-standing and highly collaborative program project is well positioned to examine these intertwined immunologic and clinical issues and to develop improved cell-based therapies for a variety of lethal hematologic malignancies. RELEVANCE (See instructions): The cumulative results of our current and proposed programmatic studies will be to increase the availability, safety and efficacy of hematopoietic cell transplant and cell-based therapies to treat otherwise lethal hematopoietic malignancies. Findings from the proposed studies can also be used to treat other potentially fatal cancers, hematopoietic, immune, metabolic and infectious disorders, and to address current barriers to solid organ transplant in children and adults world-wide.